Malonyl-CoA is an important metabolic intermediary produced by the enzyme Acetyl-CoA Carboxylase (ACC) in the body. In the liver, adipocytes, and other tissues, malonyl-CoA is a substrate for fatty acid synthase (FAS). ACC and malonyl-CoA are found in skeletal muscle and cardiac muscle tissue, where fatty acid synthase levels are low. The enzyme malonyl-CoA decarboxylase (MCD, EC 4.1.1.9) catalyzes the conversion of malonyl-CoA to acetyl-CoA and thereby regulates malonyl-CoA levels. MCD activity has been described in a wide array of organisms, including prokaryotes, birds, and mammals. It has been purified from the bacteria Rhizobium trifolii (An et al., J. Biochem. Mol. Biol. 32:414-418(1999)), the uropygial glands of waterfowl (Buckner, et al., Arch. Biochem. Biophys 177:539(1976); Kim and Kolattukudy Arch. Biochem. Biophys 190:585(1978)), rat liver mitochondria (Kim and Kolattukudy, Arch. Biochem. Biophys. 190:234(1978)), rat mammary glands (Kim and Kolattukudy, Biochim. Biophys, Acta 531:187(1978)), rat pancreatic β-cell (Voilley et al., Biochem. J. 340:213 (1999)) and goose (Anser anser) (Jang et al., J. Biol. Chem. 264:3500 (1989)). Identification of patients with MCD deficiency lead to the cloning of a human gene homologous to goose and rat MCD genes (Gao et al., J. Lipid. Res. 40:178 (1999); Sacksteder et al., J. Biol. Chem. 274:24461(1999); FitzPatrick et al., Am. J. Hum. Genet. 65:318(1999)). A single human MCD mRNA is observed by Northern Blot analysis. The highest mRNA expression levels are found in muscle and heart tissues, followed by liver, kidney and pancreas, with detectable amounts in all other tissues examined.
Malonyl-CoA is a potent endogenous inhibitor of carnitine palmitoyltransferase-I (CPT-I), an enzyme essential for the metabolism of long-chain fatty acids. CPT-I is the rate-limiting enzyme in fatty acid oxidation and catalyzes the formation of acyl-carnitine, which is transported from the cytosol across the mitochondrial membranes by acyl carnitine translocase. Inside of the mitochondria the long-chain fatty acids are transferred back to CoA form by a complementary enzyme, CPT-II, and, in the mitochondria, acyl-CoA enters the β-oxidation pathway generating acetyl-CoA. In the liver, high levels of acetyl-CoA occurs for example following a meal, leading to elevated malonyl-CoA levels, which inhibit CPT-I, thereby preventing fat metabolism and favoring fat synthesis. Conversely, low malonyl-CoA levels favor fatty acid metabolism by allowing the transport of long-chain fatty acids into the mitochondria. Hence, malonyl-CoA is a central metabolite that plays a key role in balancing fatty acid synthesis and fatty acid oxidation (Zammit, Biochem. J. 343:5050-515(1999)). Recent work indicates that MCD is able to regulate cytoplasmic as well as mitochondrial malonyl-CoA levels [Alam and Saggerson, Biochem J. 334:233-241(1998); Dyck et al., Am J Physiology 275:H2122-2129(1998)].
Although malonyl-CoA is present in muscle and cardiac tissues, only low levels of FAS have been detected in these tissues. It is believed that the role of malonyl-CoA and MCD in these tissues is to regulate fatty acid metabolism. This is achieved via malonyl-CoA inhibition of muscle (M) and liver (L) isoforms of CPT-I, which are encoded by distinct genes (McGarry and Brown, Eur. J. Biochem. 244:1-14(1997)). The muscle isoform is more sensitive to malonyl-CoA inhibition (IC50 0.03 μM) than the liver isoform (IC50 2.5 μM). Malonyl-CoA regulation of CPT-I has been described in the liver, heart, skeletal muscle and pancreatic μ-cells. In addition, malonyl-CoA sensitive acyl-CoA transferase activity present in microsomes, perhaps part of a system that delivers acyl groups into the endoplasmic reticulum, has also been described (Fraser et al., FEBS Lett. 446:69-74(1999)).
Cardiovascular Diseases
The healthy human heart utilizes available metabolic substrates. When blood glucose levels are high, uptake and metabolism of glucose provide the major source of fuel for the heart. In the fasting state, lipids are provided by adipose tissues, and fatty acid uptake and metabolism in the heart down regulate glucose metabolism. The regulation of intermediary metabolism by serum levels of fatty acid and glucose comprises the glucose-fatty acid cycle (Randle et al., Lancet, 1:785-789(1963)). Under ischemic conditions, limited oxygen supply reduces both fatty acid and glucose oxidation and reduces the amount of ATP produced by oxidative phosphorylation in the cardiac tissues. In the absence of sufficient oxygen, glycolysis increases in an attempt to maintain ATP levels and a buildup of lactate and a drop in intracellular pH results. Energy is spent maintaining ion homeostasis, and myocyte cell death occurs as a result of abnormally low ATP levels and disrupted cellular osmolarity. Additionally, AMPK, activated during ischemia, phosphorylates and thus inactivates ACC. Total cardiac malonyl-CoA levels drop, CPT-I activity therefore is increased and fatty acid oxidation is favored over glucose oxidation. The beneficial effects of metabolic modulators in cardiac tissue are the increased efficiency of ATP/mole oxygen for glucose as compared to fatty acids and more importantly the increased coupling of glycolysis to glucose oxidation resulting in the net reduction of the proton burden in the ischemic tissue.
A number of clinical and experimental studies indicate that shifting energy metabolism in the heart towards glucose oxidation is an effective approach to decreasing the symptoms associated with cardiovascular diseases, such as but not limited, to myocardial ischemia (Hearse, “Metabolic approaches to ischemic heart disease and its management”, Science Press). Several clinically proven anti-angina drugs including perhexiline and amiodarone inhibit fatty acid oxidation via inhibition of CPT-I (Kennedy et al., Biochem. Pharmacology, 52: 273 (1996)). The antianginal drugs ranolazine, currently in Phase III clinical trials, and trimetazidine are shown to inhibit fatty acid μ-oxidation (McCormack et al., Genet. Pharmac. 30:639(1998), Pepine et al., Am. J. Cardiology 84:46 (1999)). Trimetazidine has been shown to specifically inhibit the long-chain 3-ketoactyl CoA thiolase, an essential step in fatty acid oxidation. (Kantor et al., Circ. Res. 86:580-588 (2000)). Dichloroacetate increases glucose oxidation by stimulating the pyruvate dehydrogenase complex and improves cardiac function in those patients with coronary artery diseases (Wargovich et al., Am. J. Cardiol. 61:65-70 (1996)). Inhibiting CPT-I activity through the increased malonyl-CoA levels with MCD inhibitors would result in not only a novel, but also a much safer method, as compared to other known small molecule CPT-I inhibitors, to the prophylaxis and treatment of cardiovascular diseases.
Most of the steps involved in glycerol-lipid synthesis occur on the cytosolic side of liver endoplasmic reticulum (ER) membrane. The synthesis of triacyl glycerol (TAG) targeted for secretion inside the ER from diacyl gycerol (DAG) and acyl CoA is dependent upon acyl CoA transport across the ER membrane. This transport is dependent upon a malonyl-CoA sensitive acyl-CoA transferase activity (Zammit, Biochem. J. 343:505(1999) Abo-Hashema, Biochem. 38: 15840 (1999) and Abo-Hashema, J. Biol. Chem. 274:35577 (1999)). Inhibition of TAG biosynthesis by a MCD inhibitor may improve the blood lipid profile and therefore reduce the risk factor for coronary artery disease of patients.
Diabetes
Two metabolic complications most commonly associated with diabetes are hepatic overproduction of ketone bodies (in NIDDM) and organ toxicity associated with sustained elevated levels of glucose. Inhibition of fatty acid oxidation can regulate blood-glucose levels and ameliorate some symptoms of type II diabetes. Malonyl-CoA inhibition of CPT-I is the most important regulatory mechanism that controls the rate of fatty acid oxidation during the onset of the hypoinsulinemic-hyperglucagonemic state. Several irreversible and reversible CPT-I inhibitors have been evaluated for their ability to control blood glucose levels and they are all invariably hypoglycemic (Anderson, Current Pharmaceutical Design 4:1(1998)). A liver specific and reversible CPT-inhibitor, SDZ-CPI-975, significantly lowers glucose levels in normal 18-hour-fasted nonhuman primates and rats without inducing cardiac hypertrophy (Deems et al., Am. J. Physiology 274:R524 (1998)). Malonyl-CoA plays a significant role as a sensor of the relative availability of glucose and fatty acid in pancreatic μ-cells, and thus links glucose metabolism to cellular energy status and insulin secretion. It has been shown that insulin secretagogues elevate malonyl-CoA concentration in β-cells (Prentki et al., Diabetes 45:273 (1996)). Treating diabetes directly with CPT-I inhibitors has, however, resulted in mechanism-based hepatic and myocardial toxicities. MCD inhibitors that inhibit CPT-I through the increase of its endogenous inhibitor, malonyl-CoA, are thus safer and superior as compared to CPT-I inhibitors for treatment of diabetic diseases.
Cancers
Malonyl-CoA has been suggested to be a potential mediator of cytotoxicity induced by fatty-acid synthase inhibition in human breast cancer cells and xenografts (Pizer et al., Cancer Res. 60:213 (2000)). It is found that inhibition of fatty acid synthase using antitumor antibiotic cerulenin or a synthetic analog C75 markedly increase the malonyl-CoA levels in breast carcinoma cells. On the other hand, the fatty acid synthesis inhibitor, TOFA (5-(tetradecyloxy)-2-furoic acid), which only inhibits at the acetyl-CoA carboxylase (ACC) level, does not show any antitumor activity, while at the same time the malonyl-CoA level is decreased to 60% of the control. It is believed that the increased malonyl-CoA level is responsible for the antitumor activity of these fatty acid synthase inhibitors. Regulating malonyl-CoA levels using MCD inhibitors thus constitutes a valuable therapeutic strategy for the treatment of cancer diseases.
Obesity
It is suggested that malonyl-CoA may play a key role in appetite signaling in the brain via the inhibition of the neuropepetide Y pathway (Loftus et al., Science 288:2379(2000)). Systemic or intracerebroventricular treatment of mice with fatty acid synthase (FAS) inhibitor cerulenin or C75 led to inhibition of feeding and dramatic weight loss. It is found that C75 inhibited expression of the prophagic signal neuropeptide Y in the hypothalamus and acted in a leptin-independent manner that appears to be mediated by malonyl-CoA. Therefore control of malonyl-CoA levels through inhibition of MCD provides a novel approach to the prophylaxis and treatment of obesity.
Additionally, these compounds are also useful as a diagnostic tool for diseases associated with MCD deficiency or malfunctions.